LSI chips such as CPU, driver IC and memories and light-emitting devices such as LED are used in electronic equipment such as personal computers, DVD units, and mobile phones. As these electronic components are tailored for higher performance, higher speed of operation, size reduction and higher integration, they themselves generate greater amounts of heat. Elevated temperatures of the components by their own heat cause malfunction and failure to the components themselves. To suppress a temperature rise of components during operation, many heat-dissipating means and heat-dissipating members used therefore have been proposed.
In prior art electronic equipment, heat-dissipating members, typically heat sinks in the form of metal plates of aluminum or copper having a high heat conductivity are used for suppressing a temperature rise of components during operation. The heat-dissipating member conducts the heat generated by the electronic component to its surface and releases the heat from the surface due to a temperature difference between the surface and the ambient air.
For efficient conduction of the heat generated by the component to the heat-dissipating member, it is effective to fill a small gap between the component and the member with a heat conductive material. Heat conductive sheets and heat conductive grease having heat conductive fillers laden are used as the heat conductive material. Such a heat conductive material is interposed between the electronic component and the heat-dissipating member, thereby establishing a way for heat conduction from the component to the heat-dissipating member via the heat conductive material.
Since sheets are easier to handle than grease, heat conductive sheets made of heat conductive silicone rubber or the like are used in a variety of applications. The heat conductive sheets are generally divided into two categories, general-purpose sheets which are selected for ease of handling and low-hardness sheets which are selected for adhesion.
Of these, the general-purpose sheets are, for the most part, made of hard rubbers having a hardness of at least 60 in Type A Durometer hardness unit. Even sheets in thin-film form as thin as about 0.1 mm can be handled individually, but are difficult to attach to components or heat-dissipating members because they lack surface tack. One solution to this problem is a surface-tackified sheet. Specifically, a pressure-sensitive adhesive is applied to one or both surfaces of a thin heat-conductive sheet so that it may be readily attached to a component or member. However, since the pressure-sensitive adhesive applied is not fully heat conductive, the pressure-sensitive adhesive-coated sheet has a significantly increased thermal resistance as compared with the uncoated sheet. In addition, the coating of pressure-sensitive adhesive becomes a buildup of sheet thickness and also adversely affects the thermal resistance from this aspect.
On the other hand, the low-hardness sheets are formed from low-hardness heat-conductive materials having a hardness of up to 60 in Asker C hardness unit. These sheets inherently have a sufficient degree of tack to attach them to any object without a need for pressure-sensitive adhesive or the like. However, since the low hardness is accomplished by loading the material with a large amount of plasticizer or effecting crosslinking only to a low or moderate degree, a sheet formed from the material into a thin film form lacks strength and handling. The sheet must have a thickness above a certain level in order to facilitate handling. It is then difficult to reduce the thermal resistance of low-hardness sheets. In addition, the low-hardness sheets have an undesired tendency of oil bleeding to contaminate neighboring components.
The references relating to the present invention include JP-A 2005-035264, JP-A 2005-206733, JP-A 2006-182888, and JP-A 2006-188610.